This commit adds initial support for Cognito User Pools.
`UserPool` allows selecting the type of sign-in (username vs email, etc)
with options consistent with what is presented in the console.
`UserPool` also supports setting alias attributes & auto-verified attributes,
as well as setting Lambda function triggers.

A basic implementation of app clients is implemented in `UserPoolClient`.

…lem/cognito-user-pool

Some formatting & safe traversal changes (trivial).
Replacing `lambda.Function` with `lambda.IFunction`.

need to write more tests and confirm the build is still successful

this currently replaces the ability to pass lambdas in props; will fix
also still need to write tests to meet coverage threshold

updated docs & added missing tests
tests pass at 85% coverage